In general, a heat exchanger for heating or cooling a fluid is provided with a heat transfer tube through which a thermal medium fluid to be heated or cooled is circulated, and the heat exchanger is so arranged that a heat carrier fluid, such as air, is forcedly moved around the tube. The thermal medium fluid in the tube is cooled or heated by heat exchange with the heat carrier fluid through a tube wall of the tube. In such a heat exchanger using gaseous fluid as the heat carrier fluid, a heat transfer performance depends on the thermal resistance of the heat carrier fluid, such as air, and therefore, fins in a variety of forms are attached to the tubes for increasing the heat transferable contact area between the tube and the heat carrier fluid and improving the heat transfer performance.
For instance, a high-fin-tube type of heat exchanger which has spiral metal fins attached to metal tubes and the tubes disposed in a staggered arrangement or an in-line arrangement, and a fin-tube type or plate-fin-and-tube type of heat exchanger, which is known as a kind of compact heat exchanger, are incorporated in thermal medium circuits of various power plants, thermal carrier circuits of air-conditioning systems, cooling water circuits of various internal combustion engines, and so forth.
The fin-tube type of heat exchanger cools the thermal medium fluid in the heat transfer tube by heat exchange between the fluid flowing through the tube and the gaseous flow moving in an area outside the tube. The fin increases the heat transferable area of the tube so as to improve the thermal efficiency of heat exchange between the gaseous flow outside the tube and the fluid inside the tube. In such a fin-tube type of heat exchanger, a heat exchanger formed with a number of dimples or slits is disclosed in Japanese patent laid-open publication No. 8-291988 and so forth.
However, even if the heat transfer effect can be designed to be doubled by improvement of configuration of the fin, the pressure loss in the heat exchanger is caused to greatly increase on the contrary, and it difficult to overcome such a problem. Therefore, it has been understood to be difficult to realize both of augmentation of heat transfer and reduction of pressure loss of heat carrier fluid by improving the configuration of the fin.
FIG. 10 is a partial cross-sectional view of a heat exchanger which is a conventional plate-fin-and-tube type of air-cooled heat exchanger.
With respect to heat transfer tubes T extending through fins F, an air flow A is compulsorily ventilated in a direction perpendicular to the tubes T, so that the air flow A passes through fluid passages P formed between the fins F. The air flow A separates from a boundary surface of the tube T at a separation point B, when flowing rearward along the outer surface of the tube T in the passage P between the fins F. The separation point B has been considered to reside at a position rearward from a stagnation point E by an angle β, which is approximately 80°. Because of such a separation phenomenon of the air flow A, the air flow A cannot sufficiently enter the rear of the tube T, and this results in creation of a separation wake zone C behind the tube T, which is called as “dead water zone”. The separation wake zone C causes the heat transfer effect of heat exchanger to decline and the pressure loss thereof to increase.
It is a purpose of the present invention to provide a heat transfer device which can reduce the separation wake zone behind the heat transfer tube so as to improve the heat transfer effect of the heat transfer device in the heat exchanger and so forth, and which can reduce the pressure loss of the heat transfer device.
It is another purpose of the present invention to provide an air-cooled type of heat exchanger which can decrease a load of a fan for providing compulsory draft of the heat carrier fluid, thereby reducing noise of the heat exchanger during operation of the fan.
It is still another purpose of the present invention to provide a separation position control method for the heat transfer device which allows the position of separation point of the heat carrier fluid to be controlled with use of separation position control means having a simplified arrangement, thereby reducing the separation wake zone behind the tube.